Although there has been progress made during the past years, there still exist many mysteries
to be uncovered. As we have known that exotic states with normal quantum numbers (
e.g. tetraquark states [1]}, hadronic molecules [2]}, charmonium hybrids [3]}, and so on) can hardly be
distinguished from the normal ones, in order to understand exotic candidates, one should
also have a reasonable description of the normal hadron spectrum. In the charmonium sector the
low-lying states can be very well described by non-relativistic potential quark model such as the
Cornell model [4]} and Godfrey-Isgur model [5]}, [6]}, [7]}. However,
it is realized that the open channel effects would become essential for higher excited states and it
is still challenging to include such effects in a coherent way [6]}, [9]}.
